Blizzard made PvP a big part of the game, two factions, always pvp on, BGs, Arena, honor systems. Having two playable factions that are historrical in war, supporting always on pvp servers with a big honor system. Vanilla PvE was instead very limited, only a handfull dungeons for endgame pve and it took a long time until one raid was available. Raiding was always very limited, repeat the latest raid every week for at least half a year until the next raid to repeat every year. Not to mention that high end raiding in vanilla was only available for a very limited number of players. Blizzard made endgame raiding in vanilla not for tha masses, but for a very small amount of players. With the opening of BC that made PvP a even bigger part, not even 1% off all players finished Naxx, not even 10% entered Nax, not even 20% finished t2.5 content....
Dont make it look like, that blizzard made WoW PvE heavy, thats just false. It were the players and their constant river of tears that made blizzard changing their design idea, they made PvE available for the masses by lowering difficulty up to the point they brought in easy mode aka non HC, they made end game pve easier to access. They totally changed their plans about open pvp, faction war and town raids and forced pvp players out of open world pvp by honor punishment and makeing BGs more honor efficient.